HR: 0800h
AN: V31A-1416    [Abstracts]
TI: Constraints on Variable Ag:Au:Cu Ore-Metal Ratios in Felsic Arc-Magmas
AU: * Piccoli, P
EM: piccoli@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Englander, L
EM: leahe@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Candela, P
EM: candela@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AB: Silver:gold:copper ratios are variable in porphyry-type ore systems. In an attempt to better understand why, we have employed experimental techniques to determine how silver and copper, and gold from previous experiments, are sequestered in felsic magmas. To this end, we are performing sealed silica tube experiments on the equilibria among pyrrhotite-magnetite-silver alloy at 800C and at vapor pressure. Run times for the preliminary experiments were 144 hours; runs had magnetite/pyrrhotite ratio of 4. The source of silver in the runs was AgCl. Analysis of reconnaissance experiments demonstrates the stability of magnetite, pyrrhotite and a silver sulfide solid solution under the conditions of the experiments. Equilibrium concentrations of ore metals in the run products are ~3000 ppm Ag and 3500 ppm Cu in the pyrrhotite. However, the concentrations in magnetite are significantly different: 100 ppm Ag and ~20 ppm Cu. Like copper and gold (Jugo et al., 1999; Lithos), silver is concentrated in pyrrhotite relative to magnetite. The equilibrium Ag-sulfide composition in the run products is Ag53Fe8Cu3S36, with a mole fraction of Ag2S of 0.74. The log fS2 is approximated as ~ -4. The mole fraction of Ag in an ideal metal solid solution in equilibrium with an ideal model Ag2S solid solution, and a log fS2 of -4, is ~0.4. By analogy with Au, the substitution of Ag into pyrrhotite may occur as an AgFeS2 component. The substitutional mechanism for Ag in magnetite is not clear: silver may substitute as AgFe(3+)(Fe(2+))-2, but may also be present in defects in the magnetite structure. The partition coefficient (D(po/mt)) for approximately 30 for Ag. The partition coefficient for Au is higher (~120) based on the data of Simon et al. (2003; Am. Min,) and Jugo et al. (1999; Lithos). These data can be combined with data on the solubility of Ag in silicate melts to calculate mineral-melt partition coefficients. These data suggest that the role of pyrrhotite crystallization in felsic magmas is considerably more important than magnetite in sequestering Ag relative to Au. Therefore, the fractionation of magnetite relative to pyrrhotite is important in the generation of the variable silver:gold ratios in silicate melts.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3665 Mineral occurrences and deposits
DE: 3670 Minor and trace element composition
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting